Mechanistic View on the Order–Disorder Phase Transition in Amphidynamic Crystals

We combine temperature-dependent low-frequency Raman measurements and first-principles calculations to obtain a mechanistic understanding of the order–disorder phase transition of 2,7-di-tert-butylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (ditBu-BTBT) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) semiconducting amphidynamic crystals. We identify the lattice normal modes associated with the phase transition by following the position and width of the Raman peaks with temperature and identifying peaks that exhibit nonlinear dependence toward the phase transition temperature. Our findings are interpreted according to the “hardcore mode” model previously used to describe order–disorder phase transitions in inorganic and hybrid crystals with a Brownian sublattice. Within the framework of this model, ditBu-BTBT exhibits an ideal behavior where only one lattice mode is associated with the phase transition. TIPS-pentacene deviates strongly from the model due to strong interactions between lattice modes. We discuss the origin of the different behaviors and suggest side-chain engineering as a tool to control polymorphism in amphidynamic crystals.


S1 Powder X-ray diffraction measurements
Phase purity of ditBu-BTBT and TIPS-pentacene were analyzed using powder X-ray diffraction.
All the experiments were performed at room temperature. Compounds were finely ground using a mortar and pestle for phase confirmation measurement. Single crystals were mounted over the sample holder for preferential orientation analysis. For both ditBu-BTBT and TIPSpentacene, the measurements were conducted on a Panalytical Empyrean diffractometer using Cu-Kα radiation (λ =1.54178 Å). The diffractometer was set up with reflection-transmission spinner 3.0 configuration, and patterns were collected with 2θ range between 5.0 and 30.0 • . Calculated patterns were obtained from known crystal structures, ditBu-BTBT [1,2], and TIPSpentacene [2] using Powder Pattern tool on Mercury software [3,4]. The crystalline phase of ditBu-BTBT and TIPS-pentacene were confirmed to have the same phase of the known crystal structures with CSD Refcode KUDFAS01 [2] (ditBu-BTBT), VOQBIM02 (TIPS-pentacene) as it can be observed in Figure S1 (experimental diffraction pattern in red and calculated diffraction patterns in black).  Figure S2 shows the temperature dependent low-frequency Raman spectroscopy spectra of ditBu-BTBT and TIPS-pentacene from 80 K to 400 K at increments of 10 K. Figure S2: Temperature dependent low-frequency Raman of ditBu-BTBT and TIPS-pentacene. The spectra were normalized and shifted up for clarity. The temperature increment is 10 K. The red arrows indicate the order-disorder phase transition temperature.

S3 Raman spectra fitting
We fit the measured Stokes-shift Raman spectra with the product of the Bose-Einstein distribution and a multi-damped Lorentz oscillator line shape, Where ω 0,i , c i and Γ i are the position, intensity, and FWHM of each peak, respectively, ω is the measured frequency (Raman shift), T is the temperature,h is the reduced Planck constant and surements see Ref. [5]. We measure the Raman signal for selected polarization angles where different groups of peaks are most pronounced (see Figure S3). From each of them we extract the vibrational frequencies and FWHMs of the pronounced peaks by fitting the Eq. 1. Figure S4 presents the fit results for ditBu-BTBT for all low-frequency lattice vibrations, including those which we could resolve only at low temperature (colored in pink).

S4 DFT calculations
Tables S1 and S2 present the mode assignment to the experimental lattice vibration of ditBu-BTBT and TIPS-pentacene, respectively, using the DFT-calculated Raman active modes based on peak proximity and intensity. The vibrational symmetry of ditBu-BTBT were obtained from Ref. [5] and due to the low symmetry of the TIPS-pentacene crystal all lattice vibration have the same vibrational symmetry.
The agreement between the measured and calculated Raman peaks is relatively good. These values are common for this type of calculations [6][7][8][9]. The modes eigenvectors for each material are attached to this publication as media (.xsf) files. Table S1: Mode assignment: The calculated (and experimental) frequency of each of the lowfrequency Raman modes of ditBu-BTBT, alongside the calculated (and experimental) symmetry of each mode and its relative intensity. The calculation was performed on a SCXRD structure of ditBu-BTBT performed at 100 K after a full computational relaxation of atomic positions.